Prefontal cortex in long-term memory: an “interference” approach using magnetic stimulation

Abstract

Neuroimaging has consistently shown engagement of the prefrontal cortex during episodic memory tasks, but the functional relevance of this metabolic/hemodynamic activation in memory processing is still to be determined. We used repetitive transcranial magnetic stimulation (rTMS) to transiently interfere with either left or right prefrontal brain activity during the encoding or retrieval of pictures showing complex scenes. We found that the right dorsolateral prefrontal cortex (DLPFC) was crucial for the retrieval of the encoded pictorial information, whereas the left DLPFC was involved in encoding operations. This 'interference' approach allowed us to establish whether a cortical area activated by a memory task actually contributes to behavioral performance.

Human cortical EEG rhythms during long-term episodic memory task. A high-resolution EEG study of the HERA model

“…This kind of activation is also confirmed by recent studies (Rossi et al, 2001;Sandrini et al, 2004), showing that the rTMS of right DLPFC mainly impaired the retrieval of the encoded visual information. The lack of gamma synchronization on left frontal areas during encoding could be due to a minor cognitive demand requested in the encoding process as compared to the retrieval phase.…”

“…Indeed, rTMS temporarily interferes with local neural processes and can cause a transient decline of behavioral performance if the stimulated area plays a relevant functional role in the task considered (Hallett, 2000;Pascual-Leone et al, 2000;Rossini and Rossi, 1998;Walsh and Cowey, 2000). The rTMS approach has recently confirmed a functional inter-hemispherical asymmetry of dorsolateral prefrontal cortex (DLPFC) in healthy young individuals involved in long-term memory challenges, both in verbal (Sandrini et al, 2004) and visuospatial domains (Rossi et al, 2001). For the latter, the rTMS of left DLPFC temporarily disabled the encoding of complex pictures (i.e., the same material used for the current study), while the rTMS of right DLPFC mainly impaired the retrieval of the encoded visual information.…”

“…In this high-resolution EEG study, the theta (4 -6 Hz), alpha (6 -12 Hz) and gamma (28 -48 Hz) oscillations were investigated during a visuospatial long-term episodic memory task that was affected by rTMS at prefrontal areas according to the HERA model (Rossi et al, 2001). With respect to the mentioned rTMS investigation (Rossi et al, 2001), the present EEG approach allowed evaluation of the main brain rhythms of both frontal and parietal areas during the ENC and RET phases.…”

“…With respect to the mentioned rTMS investigation (Rossi et al, 2001), the present EEG approach allowed evaluation of the main brain rhythms of both frontal and parietal areas during the ENC and RET phases. The working hypothesis was that the brain rhythmicity represent a neural substrate of the hemispherical functional asymmetry as predicted by the HERA model.…”

Dorsolateral prefrontal cortex, working memory and episodic memory processes: insight through transcranial magnetic stimulation techniques

“…The interesting results from these two studies were that they did not report differences either for accuracy or for response time in the encoding/retrieval between verbal and non-verbal material. Also, Rossi et al [31] in a study with pictorial information, reported that the left DLPFC is involved in encoding operations while the right DLPFC is crucial for retrieval, providing evidence for the idea that even with non-verbal material the left DLPFC is implicated in encoding while the right DLPFC is involved in the retrieval phase. Further evidence for the crucial role of the left DLPFC was provided by innocenti et al [32] who investigated the asymmetry of the DLPFC in encoding through a paradigm that differentiates between deep (semantic) and shallow (perceptual) encoding.…”